docs: def get_train_fns(self) -> Tuple[List[str], List[str]]:
          """ Fetches the training set of the corpus.
  
          Outputs a Tuple of size 2, where the first element is a list of paths
          to input features files, one per utterance. The second element is a list
          of paths to the transcriptions.
          """
          return self.prefixes_to_fns(self.train_prefixes)